Chloride transport across the basolateral cell membrane of the Necturus proximal tubule: dependence on bicarbonate and sodium.

The transport of chloride across the Necturus proximal tubule cell was studied in the doubly-perfused kidney using conventional, chloride-sensitive and pH-sensitive microelectrodes. Lowering chloride activity in the basolateral solution results in a reduction in intracellular Cl- activity (aiCl). This reduction in aiCl is inhibited by removing either HCO-3 or Na+ from the perfusion solution, indicating that both HCO-3 and Na+ are required for Cl- movement across the basolateral cell membrane. Reducing either HCO-3 or Na+ in the basolateral solution causes an increase in aiCl. Thus changes in either Na+ or HCO-3 chemical gradients across the basolateral cell membrane significantly affect chloride movement. Changing intracellular pH by means of NH4Cl exposure results in an increase in aiCl followed by a sharp decrease when NH4Cl is removed. These changes in intracellular chloride do not occur in the absence of HCO-3. Likewise, the decrease in aiCl following NH4Cl treatment requires the presence of Na+ in the basolateral solution. We conclude that chloride is transported across the basolateral cell membrane in exchange for both Na+ and HCO-3. Our results also support the presence of a Na+/Cl- cotransport mechanism on the apical cell membrane.